The invention relates to a method of operating an extruder of a single or multiple extruder system for producing profiled sections of rubber and/or plastic mixtures.
Soft-plastic materials, such as rubber mixtures, intended for processing in extrusion systems have different properties in terms of their amenability to processing, which is substantially determined by their Mooney viscosity as defined by DIN 53 523, part 3 (issued November 1976); this will be called merely viscosity hereinafter. The differences in processability of various batches of the same type of material result from discontinuous mixture preparation in the mixing room and from the duration of storage, the storage temperature and variously long molecule chains in the case of natural rubber, among other factors. Furthermore, however, various viscosities can arise even in the rough sheet or web of material in one batch. Fresh mixtures, which have a low viscosity, can be more easily processed, that is, with a lower extruder speed or rpm and with a lower power requirement, than older batches of the same type of mixture due to the increased viscosity of the older batches. In other words, to generate a predetermined mass throughput (measured in kg/s, for example), hereinafter simply throughput, an extruder must be operated at a higher rpm and with a higher power requirement when processing an older mixture than when processing a fresh mixture of the same mixture type. Thus if the extruder rpm remains unchanged, unpredictable variations or fluctuations in the throughput will arise at different viscosities. In a single extruder, such fluctuations in the throughput can result in dimensional discrepancies in the profile. In multiple-extruder systems, fluctuations in the throughput furthermore cause a shift in the content in terms of mass of the individual components of the profile, and thus lead to undesirable loss of quality of the extruded profile.
To compensate for the influence of various processing qualities, or at least to keep such influence at a minimum, it has already been proposed (see e.g., Federal Republic of Germany Patent Application DE-OS No. 33 20 203, published Dec. 12th, 1984) that the extruder, or each extruder, be fed with a plurality of rough sheets or webs of material from different batches of the same type of material mixture so as to attain a homogenizing effect.
This known method has the disadvantage, however, that a large amount of space is required for furnishing pallets for holding the various batches. Furthermore, all that is attainable by this method is a homogeneity in the material properties by means of statistical mixing. Spontaneously occurring deviations in the statistical mean value cannot be precluded in this known method.